From the CloudBees GrandCentral (https://grandcentral.cloudbees.com), you'll have to click on the "Services" tab on the top of the page and then click on the "MongoHQ" service.
From there, click on the "View" button underneath your listed database. This will take you to the MongoHQ management screen for that db.
Now click on the Admin tab on the left-hand side of the screen and select the user tab. This will show you your username for the database, and you'll be able to set your password here.
Please note that the displayed default password is HASHED, so it's not the one that you'll use to access your database.